In preparation for incrementally migrating subclasses to deriving
from FixedArrayPtr/FixedArrayBasePtr. Once that is done for all
subclasses, this duplication will be dropped again.

Bug: v8:3770
Change-Id: I6d664997fdcb18f7c0f37183d9f920ae30f3b749
Reviewed-on: https://chromium-review.googlesource.com/c/1345325Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57702}